With increase of quantity of information treated by an information processing apparatus, such as a computer, and miniaturization of the information processing apparatus in recent years, efforts have been made to enhance the recording capacity of the information recording devices. Accordingly, recording capacity required by a magnetic recording medium (hereinafter also called “a magnetic disk”) used in the information recording devices is consistently increasing. To enhance recording capacity and improve recording performance of a magnetic recording medium, the flight height of the magnetic head must be lowered as much as possible. In particular, a perpendicular magnetic recording system, drawing much attention these days, with the magnetic flux flowing out perpendicularly to the surface of the magnetic recording medium, is required to narrow the magnetic spacing by positioning the magnetic head and the medium surface as close as possible. Recently, flight height of the magnetic head has been decreased to as low as 10 nm or lower.
A common magnetic disk comprises a substrate, an underlayer, a magnetic layer, and a protective layer, on which a liquid lubricant layer is further provided to improve durability. The thickness of the liquid lubricant layer is generally in the range of 1 nm to 2 nm. In that range of film thickness, the actual feature of the lubricant molecules are considered to be an island-like distribution scattered in the surface of the protective layer, rather than forming its own complete thin film layer. The lower layer, the protective layer, is uncovered in the area between the lubricant molecules scattering in the island-like distribution (the area absent of the lubricant), which causes several problems as described below.
A protective layer of a magnetic disk used in general is a carbon film formed by a sputtering method or a CVD method. The surface of the carbon film, being in an active state, is liable to adsorb surrounding gases or contaminants. Some types of the adsorbed gases cause corrosion together with surrounding moisture. Corrosion of the magnetic disk seriously deteriorates electromagnetic conversion characteristics and significantly degrades reliability of the HDD.
A manufacturing process for a magnetic disk in the present day technology commonly includes a tape burnishing step using a working tape after a lubricant coating step. In this step, contaminants attached in the previous steps are removed to ensure stable flight of the magnetic head. Since it is hard for the usually employed thickness of the liquid lubricant layer to completely cover the protective layer as described earlier, the tape burnishing step causes direct contact between the protective layer and the working tape. This generates frictional electric charges, which accumulate on the surface of the magnetic disk. This accumulation of frictional electric charges on the surface of the magnetic disk causes a problem of flight interruption of a glide test head or a magnetic head. If pressure at the tape working step is increased to ensure contaminant elimination in the tape burnishing step, the surface of the magnetic disk is apt to suffer from occurrence of flaws.
The above problem can be solved by wholly covering the protective layer with lubricant, that is, by forming one complete thin film of lubricant layer on the protective layer. A liquid lubricant layer having a thick thickness of 10 nm to 100 nm would completely cover the protective layer. However, such a thick liquid lubricant layer is not appropriate for a magnetic disk when the lowering of the flight height of a magnetic head is required. A next generation magnetic recording system, in which a head slides in contact with a disk, is required to raise the reliability of the magnetic disk by making use of a liquid lubricant layer and avoiding friction between the protective layer and the magnetic head. Therefore, a liquid lubricant layer with high coverage while keeping an appropriate thickness (of about 1 nm to 2 nm) is required for obtaining a magnetic recording medium of high reliability and high performance.
A typical method of forming a liquid lubricant layer is coating a lubricant on the surface of the magnetic disk. The liquid lubricant is coated generally to an average thickness of the liquid lubricant layer of about 1 to 2 nm. It is, however, difficult to sufficiently cover the protective layer by simply coating a lubricant and forming a film of about 1 to 2 nm thickness, as described previously. In a magnetic disk provided with protrusions and dents (or a texture), in particular, the dents may not be coated enough. Some attempts have been made for improving the homogeneity of the lubricant distribution occurring in the process of lubricant layer formation. For example, Japanese Unexamined Patent Application Publication No. 2003-006849 discloses a method in which a buffing process is conducted with rather high contact pressure after coating a lubricant to make the lubricant distribution homogeneous. There is another method in which a lubricant is diluted with an appropriate high solubility solvent, producing a solution with a homogeneous concentration in order to obtain homogeneous lubricant distribution in the liquid lubricant layer obtained after volatilization of the solvent.
However, a satisfactory result has not been achieved by the methods of forming a liquid lubricant layer described above in which the buffing process or the coating of diluted lubricant is conducted. Satisfactory lubricating effect cannot be obtained by the current technology with a mean film thickness of the liquid lubricant layer in a range of 1 to 2 nm. When a buffing process is conducted under high pressure, friction occurs between the buffer and the protective layer, such as a thin film of carbon, raising the risk of generating flaws on the magnetic disk. Accordingly, the buffing process imposes a severe limitation on applicable contacting material and exerting pressure in the buffing process. When a lubricant is coated using a high solubility solvent, although the distribution of the lubricant remaining on the disk surface after volatilization of the solvent can be pretty homogeneous, a complete thin film is not formed with such a quantity of coated lubricant that results in a film thickness of 1 to 2 nm. Thus, the island-like distribution of lubricant still remains.
Accordingly, there remains a need for achieving a lubricant with a film thickness of 1 to 2 nm, without the problems identified above. The present invention addresses this need.